1. Field of the Invention
The present invention relates to an apparatus for measuring a density of suspension substance (e.g., sludge, pulp, or other various substances) contained in fluid or a concentration of various dissolved substances dissolved in fluid and, more particularly, to a microwave densitometer for reliably measuring a concentration of suspension substance or the like in a wide range of concentration measurement from low to high concentrations.
2. Description of the Related Art
A densitometer for measuring a density of fluid to be measured by using an ultrasonic wave is known. Referring to FIG. 10, a measuring principle of a conventional ultrasonic densitometer will be described. The ultrasonic densitometer is typically so constructed that an ultrasonic transmitter 2 and an ultrasonic receiver 3 are so oppositely located on an interior wall of tube 1, and a fluid to be measured flow in the tube 1 is in contact with the transmitter and the receiver 3. An ultrasonic oscillator 4 is connected to the ultrasonic transmitter 2, and an ultrasonic attenuation factor measuring circuit 5 is connected to the ultrasonic receiver 3.
In this ultrasonic densitometer, an ultrasonic wave radiated from the ultrasonic transmitter 2 to the ultrasonic receiver 3 corresponding to inputting of the ultrasonic signal from the ultrasonic oscillator 4 to the ultrasonic transmitter 2. The ultrasonic wave propagated in the fluid in the tube 1 is received by the ultrasonic receiver 3. An intensity of the ultrasonic wave propagated in the fluid is attenuated in response to a density of suspension substance existing in the fluid.
The ultrasonic receiver 3 converts the ultrasonic wave thus attenuated into an electric signal responsive to its received intensity, and sends the electric signal to the ultrasonic wave attenuation factor measuring circuit 5. A calibration curve for defining the relationship between the density of suspension substance and ultrasonic wave reception intensity is set to the attenuation factor measuring circuit 5. The attenuation factor measuring circuit 5 converts the ultrasonic wave reception intensity into a concentration based on the calibration curve.
However, since the above-described ultrasonic densitometer must bring the transmitter 2 and the receiver 3 into contact with fluid flowing in the tube 1, the suspension substance is feasibly adhered to its contact surface, and hence it is necessary to periodically clean the contact surface. Particularly, when fluid such as sewage sludge flows, possibility of adherence of the suspension substance is enhanced.
Therefore, an ultrasonic densitometer of a structure in which suspension substance is not adhered is considered. This ultrasonic densitometer has an ultrasonic transmitter 2 and an ultrasonic receiver 3 located on exterior wall of the tube 1. However, the ultrasonic densitometer of this type must be reduced in thickness of a tube wall of a portion where the ultrasonic transmitter 2 and the ultrasonic receiver 3 of the tube 1 are mounted, and hence has problems in terms of its intensity and durability. Further, the ultrasonic densitometer possibly causes a measuring error by the influence of a vibration of the tube 1.
The ultrasonic wave has a very large attenuation factor in gas as compared with that in liquid. Thus, when bubbles are mixed in fluid, attenuation of the ultrasonic wave is remarkably increased as compared with that by suspension substance. As a result, there are possibilities that a concentration of the suspension substance cannot be measured and a measured result indicating higher concentration than actual concentration is obtained.
Therefore, an anti-foaming type densitometer of a structure in which bubbles contained in fluid can be removed is considered. This anti-foaming type densitometer inputs fluid to be measured into a pressurized anti-foaming chamber at a predetermined sampling period, removes bubbles by applying a pressure to the fluid to be measured, and then measures a density of the fluid to be measured. However, since the anti-foaming type densitometer has a type for sampling the fluid to be measured at each predetermined sampling period, the anti-foaming type densitometer cannot continuously measure the density of the fluid to be measured. Since the anti-foaming type densitometer needs a mechanically movable mechanism for sampling the fluid to be measured and applying the pressure to the fluid to be measured, the anti-foaming type densitometer has a problem in terms of reliability.
Since the densitometer using ultrasonic wave utilizes dispersing attenuation of an ultrasonic wave by the suspension substance contained in the fluid to be measured, this densitometer cannot measure the concentration of dissolved substance in the fluid.
Recently, a microwave densitometer in which a labor hour for cleaning suspension substance adhered to a tube can be omitted, a concentration of dissolved substance dissolved in fluid to be measured can be measured and yet the concentration can be continuously measured without sampling work in an anti-foaming chamber and which has excellent performance has been developed.
FIG. 11 illustrates a structural example of a densitometer using a microwave.
This microwave densitometer is so constructed that a microwave transmitting antenna 11 and a microwave receiving antenna 12 are disposed oppositely to a tube 1 in which fluid to be measured flows. The microwave densitometer has a first route for introducing a microwave radiated from a microwave oscillator 13 to a phase difference measuring circuit 15 through a power splitter 14, the sending antenna 11, fluid in the tube and the receiving antenna 12, and a second route for introducing the microwave radiated from the microwave oscillator 13 from the power splutter 14 directly to the phase difference measuring circuit 15. The phase difference measuring circuit 15 detects a phase delay of the microwave passed through the first route to the microwave passed through the second route as a phase difference.
In the state that reference fluid (e.g., city water) is filled in a tube, the microwave is generated from the microwave oscillator 13, and a phase delay .THETA..sub.1 of the microwave passed through the reference fluid to the microwave received without passing the reference fluid is measured by the phase difference measuring circuit 15.
Then, the microwave is generated from the microwave oscillator 13 in the state that the fluid to be measured is filled in the tube 1, and a phase delay .THETA..sub.2 of the microwave propagated through the fluid to be measured in the tube to the microwave received from the microwave oscillator 13 through the power splitter 14 is measured by the phase difference measuring circuit 15. The phase delay .THETA..sub.2 obtained by the measurement of this time is compared with the phase delay .THETA..sub.1 previously measured, and its phase delay .DELTA..THETA.=(.THETA..sub.2 -.THETA..sub.1) is substituted in a calibration curve graph to specify its concentration.
More specifically, the density or concentration X is calculated by substituting the phase delay .DELTA..THETA. in the calibration curve graph defined by a formula of the density or concentration X=a.DELTA..THETA.+b. In the formula, "a" signifies for a gradient of the calibration curve, and "b" signifies for an intercept of the calibration curve.
However, since the microwave densitometer detects the phase delay of the microwave varying in response to the concentration state of the fluid to be measured, following problem arises.
FIG. 12 shows a microwave W1 oscillated from the microwave oscillator 13, a microwave W2 having the phase delay .THETA..sub.1 received by the microwave receiving antenna 12 through the reference fluid such as city water or the like, and a microwave W3 having the phase delay .THETA..sub.2 received by the microwave receiving antenna 12 through the fluid to be measured of a certain density or concentration state.
The phase delay .THETA..sub.2 of the microwave W3 is largely varied according to the density or concentration state of the fluid to be measured. In the case where the fluid to be measured has high concentration, the phase delay .THETA..sub.2 might become an angle range of first or second rotations exceeding 360.degree..
For the convenience of description, 0.degree..ltoreq..THETA..sub.2 .ltoreq.360.degree. is called zero-th rotation; 360.degree.&lt;.THETA..sub.2 &lt;720.degree., first rotation; 720.degree.&lt;.THETA..sub.2 .ltoreq.1080.degree.; second rotation, i.e., (n-1).times.360.degree.&lt;.THETA..sub.2 .ltoreq.n.times.360.degree. is called "(n-1)-th" rotations. It is assumed that .THETA..sub.1 is set to zero-th rotation. n=integer of -1, 0 or 1, 2, 3, . . . .
As illustrated in FIG. 13, if the phase delay .THETA..sub.2 of the microwave W4 exist in an angle range of the first rotation since the concentration of the fluid to be measured is high, the phase difference measuring circuit 15 calculates "an apparent phase delay .THETA..sub.2 '" as a measured result. More specifically, the measured result indicating low concentration irrespective of the fact that the fluid to be measured has the high concentration is obtained.
If a diameter of the tube 1 is large, since a propagation route of the microwave is increased in length, the phase delay .THETA..sub.2 of the microwave W3 is increased similarly to the case that the fluid to be measured has the high concentration. If the tube 1 has a large diameter and the fluid to be measured has high concentration, the phase delay .THETA..sub.2 might become an angle of second rotations (720.degree.&lt;.THETA..sub.2 .ltoreq.1808.degree.) exceeding 720.degree..
It is as described above that the phase delay .THETA..sub.1 to become a reference is previously measured by using the reference fluid. With the phase delay .THETA..sub.1 set as a zero point, the phase delay .THETA..sub.2 of the microwave passed through the fluid to be measured is measured. However, as illustrated in FIG. 14, there arises inconvenience that, when the phase delay .THETA..sub.1 at a certain time point is a value near 0.degree., this is measured as zero point data .THETA..sub.1 and then city water is so measured as to check zero point, zero point phase delay .THETA..sub.1 is drifted to 0.degree. or less due to water temperature change or the like, and then the phase angle is advanced from zero-th rotation to -1-th rotation to apparently become an angle .THETA..sub.1 ' near 360.degree., in which the zero point is apparently largely drifted to a positive side.